Zone refining



March 18, 1969 SHQU CHU LlANG T AL 3,433,627

ZONE REFINING Filed Dec. 29, 1965 N .bfi

United States Patent O 3,433,627 ZNE REFINING Shou Chu Liang, Rossland, British Columbia,

Shaw, Trail, British Columbia, Canada, assignors to Cominco Ltd., Montreal, Quebec, Canada Filed Dec. 29, 1965, Ser. No. 517,352

U.S. Cl. 75-65 10 Claims Int. Cl. C2211 9/00 This invention relates to zone melting of fusible materials and is particularly directed to an improved method and reciprocating apparatus for completing a zone melting operation in the purification of fusible materials and/or in the growth of crystals of fusible materials.

In the zone melting of fusible materials, it is well known as described in Canadian Patent No. 532,501 issued Oct. 30, 1956 to W. G. Pfann to form a small molten zone Within an elongated body of fusible solid material and to traverse the length of said body with said molten zone. Solutes added to the material can be controlled, that is the concentration may be uniformly distributed throughout the length of the body or increased or decreased as desired in portions of the body, or undesirable solutes initially present in the material can be segregated and collected in one end of the body for subsequent removal. Further, the body can be solidified to form a single crystal or a few large crystals superior in quality and in size to the crystals produced by uncontrolled crystallization.

The technique of controlling the distribution of the solutes throughout the length of the body is known as zone leveling, the technique of collecting the solutes in one or the other end or in both ends of the body is known as zone rening and the technique of growing crystals in the body is known as crystal growing. The process of zone melting includes the processes of zone leveling, zone refining and crystal growing. By these techniques a material suitable for use as semiconductor material having a high degree of purity with closely controlled quantities of solutes which impart predetermined electrical characteristics to the material can be produced.

In the conventional practice of zone refining, it is customary to form a plurality of discrete, linearly equispaced molten zones along the length of the body to be treated by means of a plurality of radiant, inductive or the like heaters which are slowly moved in one direction along the body a distance substantially equal to the spacing between the molten zones or to an integral multiple of said spacing and then are rapidly returned to their starting points. These starting points are substantially coextensive with the next succeeding molten zones (except, of course, in the case of the end heater which starts ya new zone) for repeating the cycle and continuously advancing each and George molten zone from one end of the elongated body to the other. We have found that when the desired number of passes in a zone reliner has been completed and all the heaters are shut down simultaneously, the molten zones freeze in their respective positions resulting, firstly, in the solidication of the solute material in the discrete zones yielding a material with a plurality of zones having separate and distinct chemical discontinuities and, secondly, in the production of a body having irregular and erratic physical dimensions and an unattractive appearance. To overcome these disadvantages, it has been deemed necessary, especially in the production of single crystals, to employ one molten zone only traversing the body from one end to the other. This technique results in a slow and commercially impractical operation.

We have found that the disadvantages inherent in conventional techniques of zone rening can be substantially overcome by sequentially tie-energizing the plurality of equi-spaced heaters one at a time substantially concurrently with each rapid retraction of the said heaters for ACC continuously advancing each molten zone towards the finishing end of the body treated.

It is a principal object of the present invention, therefore, to provide a method and apparatus for terminating a zone rening operation for the production of high purity materials.

Another object of the present invention is the provision of an apparatus including electrical circuitry which is reliable in operation and amenable to automation.

These and other objects of the present invention, and the manner in which they can be attained, will become apparent from the following detailed description of the apparatus taken in conjunction with the drawing, wherein:

FIGURE 1 is a schematic-pictorial illustration, partly cut away, of the apparatus including circuitry of the present invention; and

FIGURE 2 is a schematic illustration of a portion of the circuitry shown pictorially in FIGURE l.

Like reference characters refer to like parts throughout the description of the drawing.

With reference to FIGURE l, boat 10 having a fusible body 12, eg., metal, to be treated therein is positioned within tube 14 having a desired atmosphere, e.g., hydrogen or argon. Tube 14 is rigidly secured in the desired stationary position by clamps 16. A carriage 18 mounted on wheels 2t) for reciprocal movement below tube 14 longitudinally thereof is provided with external power means, not shown, for reciprocating said carriage slowly in one direction and rapidly in the opposite direction. The plurality of annular heaters 22a-22j, supported by carriage 18 are spaced an equal distance apart on said carriage and slightly spaced from tube 14 to allow for the reciprocal movement of carriage 18; the amplitude of reciprocal movement of carriage 18 in this embodiment being substantially equal to the spacing of the heaters.

Each of heaters 22a-22j has a resistance element 26 or the like heat source means for heating body 12, said resistance elements each being in communication with an electrical power supply as will be described hereinbelow with reference to the circuitry of heater 22j. One lead 28 from the resistance element is connected to terminal 30 of transformer 32, e.g., a variable autotransformer, which is in communication with one side of the line 11S-volt lead 34. The opposite lead 36 from the said resistance element is connected to the terminal 38 of said transformer 32 by way of switch 40j and lead 42 thereby permitting control of the voltage to heater 22j by adjustment of the said transformer 32. The third terminal 44 of said transformer 32 is connected to line lead 46. Each of the plurality of heater leads is likewise connected to a transformer via a corresponding switch 40u-40j, said switches being actuatable by cams 48u-48j mounted on output shaft 50 of a prime mover, Afor example motor 52, for rotatable movement therewith. Cams 48a-48j .are so formed that closed switches 40u-40]' are opened in sequence from switch 40a to switch 40]' as said cams are progressively rotated by shaft 50. Normally de-energized motor 52 is connected to line lead 34 by lead 54 and to opposite line lead 46 via lead 56 to a single-pole singlethrow switch 5S, lead 60, single-pole double-throw switch 62, leads 64 or 66, single-pole double-throw switch 68, lead 99, switch 74 and lead 70.

The operation of the present invention will now Ibe described with reference to FIGURES 1 and 2. Carriage 18 is reciprocated longitudinally below tube 14 from the position designated A to the position designated B at the desired speed to effect the movement of molten zones 72 in body 12 from one end of said body to the other. The forward and rearward movement of carriage 18 is controlled by a pair of limit switches 71, 73 which are in communication with a prime mover such as an electric motor with gear reducer and a suitable clutch which permits the forward movement of the molten zones 72 at a desired rate with rapid return of the heaters to the starting position. When it is desired to start the sequential de-energization of the heaters, the operator or a previously set timer closes normally open single-pole single-throw switch 74 thereby readying the circuit to motor 52 for energization of said motor upon actuation of switch 62 by carriage 18 at its point B of maximum forward travel. Switch 62 is actuated to the position shown by the broken line in FIGURE 2 closing the circuit t line lead 46 via leads 56, 60, 66 and 70, causing motor 52 to be energized and rotating shaft 50 in a counterclockwise direction as viewed in FIGURE l. Cam 76 has circumferentially spaced top lands and roots, each top land completing a circuit to motor 52 during movement of the carriage from A to B and the roots completing a circuit to the motor upon movement of the carriage from B to A. Cam 76 is rotated with shaft 50 actuating switch 68 to the position shown by the broken line in FIGURE 2 thereby interrupting the circuit to the line lead 46 and de-energizing motor 52. As carriage 18 begins its return to the start position A, switch 62 is returned to the position shown by the solid line thereby again closing the circuit to the line lead 46 and re-energizing motor 52 to rotate shaft 50 and the cams mounted thereon. Cam 76 again actuates switch 68 upon rotational movement of said cam through a predetermined angular displacement, returning switch 68 to the position shown by the solid line to open the line circuit and de-energize motor 52. The switches 62 and 68 are now in the same position they were in before switch 62 was actuated by carriage 18 at its point of maximum forward travel, point B.

Cam 48]' is rotated by this two stage energization of motor 52 to a position which permits switch 40j to open and de-energize resistance element 26 in heater 22j. Upon the return of carriage 18 to point A, heater 221' is positioned over the molten zone formerly under heater 22j, and heater 221' advances said molten zone as carriage 18 moves to its forward position B. At position B, the carriage actuates circuitry to de-energize heater 22 in the manner described for heater 22j. When the carriage returns to point A, heater 22h takes up the molten zone advanced by heater 221'. In normal zone refining a new zone is formed at the beginning of each advance of the carriage 18, i.e., under heater 22j, but after sequential de-energization of the heaters has been initiated no new zone is formed.

By this sequential de-energization of the heaters concurrently with the return of the heaters to their start position by the reciprocal movement of the carriage, the last formed molten zone is conveyed through and traverses the body in a continuous manner from one end to the other.

Upon the rotation of cam 48a to an angular position opening the last heater switch 40a, earn 84 opens switch S8 thereby preventing continued energization of motor 52 and rotation of the cams to positions which would close the heater switches and re-energize the heaters. By closing normally open single-pole single-throw switch 80 in by-pass lead 82, motor 52 is energized thereby rotating cam 84 to a position closing switch 58 and switches 40u-40]'. The equipment and circuitry are thus placed in condition for initiating the sequential de-energization again. Switch 74 is opened of course, for normal zoning operations and kept open until the cle-energization procedure is to be initiated.

A voltrneter 86 can be positioned between leads 28 and 36 to the heater for determination of the transformer output voltage and an ammeter 88 in series with line 90 having a single-pole double-throw switch 92 for determination of current iiow to the heater. The voltmeter and ammeter are used as relative indicators only.

It will be understood, of course, that modifications can be made in the preferred embodiment of the invention described herein without departing from the scope of the invention as defined by the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. In a method of treating an elongated body of fusible solid material which comprises forming a plurality of discrete, linearly equi-spaced molten zones in said body with a plurality of correspondingly numbered spaced heaters forming heating stages, advancing said molten zones slowly at a uniform rate through said body from one end to the other end by reciprocating the plurality of energized heating stages at the said uniform rate in the direction of advance and a relatively rapid rate in the direction of return, the distance of reciprocation not exceeding the distance between the adjacent heaters, the improvement which comprises sequentially de-energizing said heating stages one at a time substantially concurrently with the rapid return of said heating stages for continuously advancing each molten zone to the finishing end.

2. In a method as claimed in claim 1, the amplitude of reciprocal movement of said heating stages being substantially equal to the spacing of said heating stages.

3. In a method as claimed in claim 2, said heating stages each comprising an electrical heating element formed annularly about said vessel.

4. In a method of zone heating a fusible material disposed in a substantially horizontal, elongated vessel having a head end and a tail end and a plurality of linearly equi-spaced heating stages reciprocated longitudinally in proximity to said vessel slowly towards said tail end and rapidly towards said head end for continuously conveying a plurality of discrete molten zones through said material, the improvement which comprises automatically sequentially de-energizing said heating stages one at a time substantially concurrently with the rapid reciprocal movement towards the head end for conveying each molten zone to the `said tail end.

5. In a method as claimed in claim 4, the amplitude of reciprocal movement of said heating stages being substantially equal to the spacing -of said heating stages.

6. In a method as claimed in claim 5, said heating stages each comprising an electrical heating element formed annularly about said vessel.

7. An apparatus for treating an elongated body of fusible material in a vessel which comprises means for heating said body at intervals along the length of said body for forming discrete, linearly equi-spaced molten zones therein, means for advancing said heating means at a slow rate along said body a distance at least equal to the spacing of said heating means and returning said heating means at a rapid rate the same distance for continuously advancing said molten zones from one end of the body to the other, and means for sequentially shutting down said heating means one at a time substantially concurrently with the return of the heating means for advancing each molten zone through the body.

8. In an apparatus as claimed in claim 7, said means 'for advancing and returning said heating means comprislng a carriage mounted for reciprocal movement, motor means operable for positively advancing said carriage and means for returning said carriage, and limit switch means in communication with said motor means and actuatable by said carriage for de-energizing said motor means upon completion of advance of the carriage and energizing said motor means upon completion of return of the said carriage.

9. In an apparatus as claimed in claim 8, said means for sequentially shutting down said heating means comprising a power supply, a prime mover in communication with and energizable by said power supply having an output shaft journalled for rotational movement therein, a plurality of cams rigidly mounted on said shaft adapted for rotational movement therewith, a plurality of switches in series with circuit means independently communicating said heating means with the power supply and operatively engaged by said cams for progressively de-energizing said heating means upon cumulative rotation of said cams, a pair of cascaded single-pole doublethrow switches in series with said power supply and prime mover adapted such that one of said single-pole double-throw switches is actuatable -by the advance and return of the carriage and the other of said single-pole double-throw switches is actuatable by one of said cams for progressively rotating said shaft at the completion of each advance of the carriage such that the said heater switch means are progressively actuated for sequentially de-energizing said heating means.

10. In an apparatus as claimed in claim 9, actuation of one single-pole double-throw switch communicating said prime mover with said power supply upon completion of advance of said carriage and energizing said prime mover for rotation of the shaft and cam operably connected to the other single-pole double-throw switch for de-ener-gizing said prime mover, and initiation of return of said carriage reversing said first single-pole double-throw switch for re-energizing said prime mover for further rotation of the shaft and cam operably connected to the said other single-pole double-throw switch for reversing said second switch and de-energizing said prime mover, each two-step angular rotation of said shaft being suicient to progressively actuate said heater switch means and sequentially de-energize the heating means.

References Cited UNITED STATES PATENTS 2,719,799 10/ 1955 Christian 23-301 NORMAN YUDKOFF, Primary Examiner'. EUGENE P. HINESJ Assistant Examiner.

U.S. C1. XR. 

1. IN A METHOD OF TREATING AN ELONGATED BODY OF FUSIBLE SOLID MATERIAL WHICH COMPRISES FORMING A PLURALITY OF DISCRETE, LINEARLY EQUI-SPACED MOLTEN ZONES IN SAID BODY WITH A PLURALITY OF CORRESPONDINGLY NUMBERED SPACED HEATERS FORMING HEATING STAGES, ADVANCING SAID MOLTEN ZONES SLOWLY AT A UNIFORM RATE THROUGH SAID BODY FROM ONE END TO THE OTHER END BY RECIPROCATING THE PLURALITY OF ENERGIZED HEATING STAGES AT THE SAID UNIFORM RATE IN THE DIRECTION OF ADVANCE AND A RELATIVELY RAPID RATE IN THE DIRECTION OF RETURN, THE DISTANCE OF RECIPROCATION NOT EXCEEDING THE DISTANCE BETWEEN THE ADJACENT HEATERS, THE IMPROVEMENT WHICH COMPRISES SEQUENTIALLY DE-ENERGIZING SAID HEATING STAGES ONE AT A TIME SUBSTANTIALLY CONCURRENTLY WITH THE RAPID RETURN OF SAID HEATING STAGES FOR CONTINUOUSLY ADVANCING EACH MOLTEN ZONE TO THE FINISHING END. 